Deposition of material onto a mosaic screen through a stencil



J1me 1952 J. D. MCGEE ET AL DEPOSITION 0F MATERIAL ON TO A MOSAIC SCREEN THROUGH A STENCIL Filed Dec 6, 1946 Patented June 10, 1952 DEBQSITI-QN QF MATERIAL ONT-D A MOSAIC SCREENTHROUGHA STENCIL James Dwyer McGee, London, and Walter Ernest Turk, Hayes, England, assignors to Electric .& Musical Industries Limited, Hayes, England, a summer of fittes Bri n hl h ioat onll eccmbcr 6, 1946, S rial N 71. lnGreat Britain October 6, 1945 s tion lglnhliclaw coo, August 8,.l9.46 Patent expires October 6,1965

3 Claims. 1

h in ention relates to the deposition o ma erial on to a carr through a stencilobjeot of he invention is to provide an improve mode of obtaining intimate contact between the stencil and the carrier.

The invention has particular reference to the Production of mosaic screens of the kind comprisinga multiplicity of mutually insulated conducting elements, such screens being used for example in cathode ray tubes employed in television cameras. It is known to prepare such screens by .depositing thev material from which the elements are to b formed n a carrier ithlollgh .a fine wire mesh, but difficulties have been encountered in obtaining good contact bet e n the me h nd the carrier over the whol ar a ver h ch m terial is to be deposited, owing to he fiexih ty nd fineness of said. mesh. which ma ho f t e o d oi a few thou andths of a inch n thickn s w th the resu t that material tends to pe trate u erneath the wires of the mesh and thus to afiect adversely the mutual nsulat on o he el men Accordi t ne fea ure of the invention the desired close contact between the stencil and the carrier is produced by the application of suitable electrostatic force to the stencil so as to. urge it rd the c rr er- The i ve o can be applied with avantase to he man ac ur mosa c screens from materials which cannot conveniently be made by depositing a thin layer of the material which is to form the mosaic elements and then heating the layer to cause it to form globules as in the. case of a mosaic screen having silver elements. not practicable for example to use this latter method the man ctu e of mosaic screens om antimon or bismuth or components of antimony or b smu h with othe metals for ample palladium, ruthenium, platinum, iridium and osmium, whereas eflicient screens can be made from these materials by depositing them on to a carrier through a stencil and activating them to photo-sensitivity by subjecting them to the action of an activating substance, for example caesium, ribidiuxn or other alkali metal. A still further advantage of the application of the invention to the manufacture of mosaic screens for cathode ray tubes is thatit facilitates the formation of a screen in situwithin thetub'e envelope and this is of particular importance in the ase oi he above-mentione materials since the 'efi o enc i c a ed. screens ade iro hem is i pai d. by exposure. of the una tivated elements to he a mos here. In, the form It is and activation of the elements within a time envelope gases or vapours adversely affecting-the mosaic elements can'be excluded'bylthe conventionalevacuation of the envelope or by providing the envelope with a filling of inert gas. The invention can also be applied to the manufacture of mosaic screens from other materials, for example silver.

Moreover in the manufacture of photoelectric mosaics in which the deposition of the material to form the mosaic elements is efiected within a closed vessel, not only can the stencil be kept in close contact with the carrier without the use of clips or other securing means, but it also enables the stencil to be readily released from the carrier when desired, itbeing merely necessary .for this purpose to cease the application of the electric field.

In the application of the invention to the manufacture of a mosaic screen fora cathodeiray tube, using a stencil for the formation of the mosaic elements, .it may be desirable that the stencil should be removed .from the envelope when it is .no longer required. For this purpose the envelope may be provided with .a side tube through which the stencil may be withdrawn, a seal then being, made at the junction of the side tube with the envelope, the side tube then being removed. The side tube may also serve to .enable the stencil to be introduced into the envelope prior to the formation .of the .mosaic elements and for this purpose the stencil may be rolled up one end of a rod and passed through the side tube. the latter being so arranged that when the rod is rotated in the appropriate direction the stencil unrolls and hangs from the rod in front of the carrier plate. Electrostatic force is then applied to the stencil to urge it into intimate contact with the carrier and when the deposition of the elements is completed, the electrostatic force is removed, and the .rod is rotated so as to wind up the stencil and is thenwithdrawn with thestencll into the side tube. To facilitate, rotation. of the rod and its withdrawal, the. rod may be provided with an armature of magnetic material to which appropriate magnetic forces are applied for rotating the rod or withdrawing it into the side tube.

Figure 1 discloses a sectional view of atube and structure used in the method of making. a mosaic screenv according to our invention.

Figure 2 isa sectional view of Figure l on line AA.

Figure 3 is a view of a composite stencil and mask according to our invention. 7

In one method of makinga photoeleotricmosaic screen in accordance with the invention for a cathode ray tube for use in television transmission, a flat rectangular carrier I of glas or mica or other suitable dielectric material is provided on one side with a metal grill or an opaque or a thin transparent layer of metal I2 which serves as the signal plate of the mosaic screen. The carrier I0 is mounted within a cathode ray tube envelope I4 with the metal layer on the side of the carrier I0 remote from the electron gun I 6 of the tube. The carrier I0 may be normal to the axis of the electron gun It or inclined with respect thereto as in some types of television transmitting tubes. The envelope I4 is provided with a side tube I8 projecting laterally therefrom and so arranged that the axis of the side tube I8 when produced extends across and slightly in front of the carrier. A rectangular flexible stencil 20 in the form of a fiat mesh of fine metallic wires for example of silver, nickel, or aluminium is attached at one edge to a metal rod 22. If an aluminium mesh is used it is preferably subjected to an anodising treatment prior to being used. The area of the mesh 20 is slightly greater than that of the desired mosaic. The mesh may have, for example, from 200-400 meshes per inch and should have a suitably small shadow ratio, for example, less than 30 per cent. The rod 22 which is provided at one end with a magnetic armature 24 comprising a pair of pole pieces, one on each side of the rod, is rotated so that the mesh is wound up on the rod. The rod 212 is then passed through the side tube I8 until the rolled-up mesh 20 extends across the carrier plate I0. The side tube I8 is then sealed, and the envelope M is connected to a pump and evacuated and degassed by baking it at a temperature of about 400-500" C. The rod 22 is then rotated by suitably manipulat ing a magnet outside the side tube so as to rotate the armature 24, until the mesh 20 hangs down in front of the carrier I 0. The tube I is then arranged with its axis vertical so that the mesh hangs away in a curve from the carrier plate. A suitable electric potential is applied between the mesh I0 and the signal plate I2, producing an electrostatic pull between them which pulls the mesh into firm contact with the carrier plate I0. For this purpose a potential diiferenoe, for

example, of the order of 500-1000 v. is applied between the mesh 20 and signal plate through n the intermediary of leads 26 and 28 sealed into the envelope and connected to the rod 22 and the signal plate I2, respectively. It will be appreciated that first the part of the mesh 20 adjacent the rod and then successive areas of the mesh will be drawn into contact with the carrier plate, so that the mesh is spread evenly over and adheres flat to the carrier plate I0. Antimony is then evaporated from a heater element (not shown) within the envelope and becomes deposited on those -.3

parts of the carrier l0 which are left exposed by the mesh 20, the envelope I4 being still connected to the pump. There will be little likelihood of the penetration of stibide under the wires of the mesh owing to the close contact between .i the mesh and the carrier plate. The envelope I4 the envelope gently in order to release the mesh from the carrier or apply the same high potential to both mesh and signal plate to mutually repel them. The rod 22 is then rotated, by using a magnet a before, so as to wind up the mesh 20 on the rod, and the armature 24 is drawn along the side tube so that the rod with the mesh thereon is withdrawn into the side tube I8. The side tube is then sealed off at a constriction 30 joining it to the envelope I4 and removed with the rod and mesh. There is left on the carrier a mosaic consisting of a multiplicity of mutually spaced elements of stibide, and said elements are then activated by the deposition of caesium, which may be distilled from a side tube, as described in British Patent No. 513,523, or, as is preferred, released by heating from a capsule carried by a heating filament within the envelope.

The antimony is preferably arranged to be shielded during the above-mentioned degassing operation by withdrawal into a side tube. If the antimony is exposed in the envelope during the degassing operation the baking temperature should not exceed 300 C.

It may happen that durin the evacuation and/or baking of the envelope and the components therein and prior to the formation of the mosaic elements the surface of the carrier becomes contaminated with minute quantities of antimony evaporated from the above-mentioned source or sources, or with other undesirable matter. This may subsequently impair the mutual insulation of the mosaic elements. In order to avoid such contamination of the carrier, means may be provided for screening it during the evacuation and/or baking of the envelope. For example, as shown in Figure 3, themesh 20 referred to above may be attached indirectly to the rod 22 through the intermediary of a mask of metallic foil or other suitable material of about the same dimensions as the mesh. Examples of suitable materials are gold, platinum and aluminium foil. The mask 40 is attached at one edge to the rod 22 so as to hang from it and the mesh 20 is attached to the lower edge of the mask, and the rod is inserted into the envelope with the mask and mesh Wound up on it. Prior to the evacuation of the tube the rod 22 is rotated so as to unwind .both the mesh 20 and the mask 40 so that the latter hangs down in front of the carrier and electrostatic force is applied to the mask 40 so as to pull it firmly into contact with the carrier plate. The mask is held in contact with the carrier in this manner during the evacuation of the envelope and the main baking operation, so as to protect the surface of the carrier from contamination. When the antimony is to be deposited on the carrier the electrostatic force is removed and the rod 22 is rotated so as to wind up the mask 40 on it and to bring the mesh 20 in front of the carrier I0. Electrostatic force is then applied to the mesh 20 to pull it into contact with the carrier as previously described, and the deposition of antimony and subsequent operations are then carried out in the above dcscribed manner.

The envelope I4 is kept connected to the pump following the degassing and is sealed off after activation of the stibide mosaic elements with caesium, so that the whole process of forming and activating the mosaic screen takes place in vacuo. If preferred, the mesh may be removed through an opening in the envelope while the latter is kept full of an inert gas to prevent the 5 ingress of air which may have a deleterious effect on the stibide elements.

In a modification of the above method the mesh instead of being introduced into the envelope as above described may be clipped to the carrier prior to the insertion of the carrier into the envelope, electrostatic force being used as above described to produce close contact between the mesh and the carrier at the appropriate time. When the formation of the mosaic elements is completed the mesh may be withdrawn from the envelope through a side tube or the tube may be turned over so that the mesh hangs down over the signal plate.

What we claim is:

1. The process of making a mosaic screen by depositing conductive material through a stencil onto a carrier, said process comprising the steps of, applying an electrostatic difference of potential between said carrier and said stencil prior to the deposition of said conductive material, depositing said conductive material through said stencil onto the exposed surface of said carrier, applying an equal high electrostatic potential to both said stencil and carrier to mutually repel them to release the stencil from the carrier after deposition of said conductive material.

2. The method of processing an electron discharge device having therein a carrier and a mask mounted adjacent thereto, said method comprising the steps of, applying an electrostatic difierence of potential between said mask and said carrier to urge the mask into close contact with said carrier during the processing of said discharge device, processing said discharge device,

applying an equal high electrical potential to REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,077,442 Tedham et al Apr. 20, 1937 2,087,683 Farnsworth July 20, 1937 OTHER REFERENCES Static Electricity, by Hobart Mason, 1904 Ed, McGraw Publ. Co., pgs. 2 and 54. 

3. THE PROCESS OF MAKING A MOSAIC SCREEN BY DEPOSITING MATERIAL INTO A CARRIER THROUGH A STENCIL, SAID PROCESS COMPRISING THE STEPS OF, APPLYING AN ELECTROSTATIC DIFFERENCE OF POTENTIAL BETWEEN SAID CARRIER AND SAID STENCIL, DEPOSITING SAID MATERIAL THROUGH SAID STENCIL ONTO THE EXPOSED SURFACE OF SAID CARRIER, AND SEPARATING SAID STENCIL AND CARRIER. 